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Here's a quote from TCP/IP First Step - Book

Router's role is to figure out what to do with IP packets that it receives. One thing routers can't generally do is function as a LAN. They can interconnect LANs, but they are not LANs in and of themselves.

Why can't routers function as a LAN yet they interconnect LANs? What does it mean?

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This makes absolutely no sense in real world applications. – phil Jan 16 '12 at 9:37
It makes sense, phil, if one simply reads the immediately preceding paragraph of that book. – JdeBP Jan 16 '12 at 14:38

The function of a router is to facilitate communication between two or more different networks by routing packets. Although if it has multiple ports in the same network , it does switch packets between them.

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eg. A wifi router with multiple hosts connected acts as switch for the hosts(LAN).

Its basic function is to route packets between two networks, internal and the internet.

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A router by definition routes - it moves packets between networks.

A switch is what you use for a network, it passes packets between devices on the local network - the LAN. Switches often have the ability to support VLANs. Ports are allocated to a VLAN and cannot communicate with ports allocated to different VLANs, which effectively creates multiple virtual LANs (vlans).

A "layer 3" switch, is a switch that can route between VLANs. So it is both a switch, and a router. The routing aspect of a layer 3 switch is usually limited, and not intended to be used as a WAN router for example. Its role is usually to route packets between the VLANs it has configured.

Some routers, such as domestic routers, come with switchports. So this is effectively a router with a switch.

These last type of routers can definitely support a LAN. However, it only has a few switchports, so couldn't support a large LAN.

The article is really referring to the absolutes of the term router, which is blurred in reality to target different needs.

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A router is meant to deal with OSI level 3 traffic which deals with IP networks and IP addressing. What they are trying to say, is that a router usually has a port that is connected to a switch that will have multiple ports. That switch is usually a network unto itself (let's say 192.168.0.x). All the computers on the LAN are plugged into the switch and all inter-LAN traffic is handled by the switch. If a packet needs to go to another network (say 176.12.7.x) it is then sent to the router which will decide what the best route is for that packet to take out one of it's other ports based on it's known network map. So your confusion is probably because you have a home "router" that connects your home LAN. What you are seeing in use is really a combined router/switch.

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